Entanglement is a quantum mechanics phenomenon. It’s been called “spooky action at a distance” because we can see that – on tiny scales at least – particles that were once together seem to have an instant effect on each other even when separated by vast distances. Take two electrons, move them apart, change one of them a little bit, and the other one seems to change correspondingly and instantly.
We don’t yet know why or how quantum entanglement happens. It’s also not clear why we don’t seem to see the effects on the everyday scale of the universe we can directly observe. But entanglement is a solid, proven, never-failing fact. Until recently, however, it’s only been seen in things like the spin of electrons, or the polarisation of photons.
A few years ago the Institute for Quantum Optics and Quantum Information in Austria did some tests – now being published in Nature – that demonstrate entanglement at the atomic level. This is a step up the mechanical chain, a level closer – though still pretty far from – the apparently classical world we’re familiar with.
I think it’ll be really interesting to discover at what point on the subatomic-to-breadbox scale that these quantum entanglement effects disappear (if they really do at all).